The current wavelength selective switches mainly include the following three forms: MEMS (Micro-Electro-Mechanical System)-based switches, LCOS (Liquid Crystal on Silicon)-based switches, and LC (Liquid Crystal)-based switches. Such three types of wavelength selective switches generally include an I/O (Input/Output) collimator, a beam expanding device, a light splitting device, a focusing device, and an attenuation switching device arranged in this order along an optical path. The I/O collimator may be formed by an array of fibers and a corresponding array of micro lens, or it may be formed by directly assembling a plurality of single collimators. The beam expanding device may include a set of prisms, or a plurality of lenses with respective predetermined focal lengths. The light splitting device generally includes a dispersion grating, which may be a transmission grating or a reflection grating. The focusing device may include a set of lenses or a concave mirror to focus light of various wavelengths from respective ports onto a focal plane. The attenuation switching device may be located at or near the focal plane to adjust optical power of and select output ports for respective wavelengths. Currently there are three types of attenuation switching devices that are commercially available: MEMS-based attenuation switching devices, LCOS-based attenuation switching devices, and liquid crystal plus birefringent wedge-based attenuation switching devices. In addition, if linear polarized light is required for any of the above optical components, a birefringent polarizer should be added into the optical path.
Among the three types of wavelength selective switches, the LC-based switch is favored because of its advantages such as good stability, mature process, and low cost. The LC-based wavelength selective switch includes a liquid crystal attenuation switching device for selection of output ports and adjustment of optical output power, which further includes a liquid crystal cell for attenuating and one or more liquid crystal cells for switching.